I'm not entirely sure what "Cycling Event" signs are actually supposed to achieve.
I suppose as a motorist they'd mostly tell me that there's little point in trying to overtake this little group of MAMILs, as there's only going to be another cluster of them round the corner.
On CTT events they're mainly used at junctions where the riders have priority - thus IN THEORY reducing SMIDSYs. It is a fact that more RTAs occur at junctions than the number of hit-from-behind incidents. HOWEVER no statistics show a reduction in accidents from putting these signs out
It largely depends on what sort of event it is. On a mass start cycling event, the signs are positioned to warn
oncoming traffic and traffic coming out of junctions. In these events, the ability to overtake the group of riders (who are largely unlikely to be MAMILS) will be severely restricted by the race convoy vehicles. They are also used at junctions where marshals will be stopping traffic to allow to race to proceed where they wouldn't normally have priority. In TTs they are primarily used at junctions, and in triathlon they are used randomly.
In the case of horse riders, when approaching a junction with a big 'caution cyclists, race in progress' sign on it, most horse riders will decide that they may want to take a slightly different, more enjoyable route that day, because by and large, their rides are not determined by needing to go from A to B. In this case, the rider had made a point that she hadn't known about the event, and hadn't seen any signs to indicate she was on the race route. Presumably if she had known, she would have gone somewhere else, or gone at a different time.
Also, not putting out the signs is indicative of either not doing a full risk assessment, or not complying with one. Whilst many people wouldn't think about livery stables, the risk assessors for a British Cycling event certainly do. In higher risk locations, organisers are instructed to visit livery stables to notify them. Race briefings will include rider behaviour and not being a dick. Whilst the specific actions on the video clip are individual irresponsible behaviour, race organisers and governing bodies have a role to play in safety of all road users during their events.